A conventional multi-layer PCB includes conductive material in holes of an insulating layer for electric conduction between two conductive layers respectively overlying and underlying the insulating layer. Depending on the disposition ways in the board, three types of holes, including through holes, blind via holes and buried via holes, are commonly known in the art.
Conventionally, holes are created by drilling the insulating layer with a machine or laser, and copper is deposited on inner walls of the holes by way of electroplating, thereby forming the conductive holes. As known, an electroplating process involves long processing time, high cost, serious contamination and high energy consumption. Aside from, since it is hard to fill the entire space inside the holes with copper by way of electroplating, there are likely voids existing in the holes, particularly in the center and top/bottom of the space, so as to underlying affect the effective area of the overlying and underlying copper lines (pattern) at these sites. Furthermore, if the resulting device involves a relatively large intensity of load current, a relatively large size of each single hole would be required in consideration of minimizing resistance. However, more and larger voids might be formed in a larger hole. Conventionally, resins are additionally used for filling voids. The conductive holes thus occupy a significant area of the circuit board. Resin filling would hinder miniaturization of the circuit board, lower reliability, and increase procedures, time and cost. Moreover, the intensity and density distribution of the load current still vary with the copper electroplating thickness on the inner wall of a single hole and the cross-sectional area of the single hole. To prevent the electroplated copper on the inner wall of a single hole from stripping, extensive electroplating would be needed to form a copper ring of 0.5 mm wide or more surrounding the top/bottom openings of the hole.
Another prior art is related to a multi-holes structure, which is advantageous in reduced resistivity, but on the other hand, might cause problems in layout. For example, long electric traces might be required in the layout in order to bypass the conductive holes. Furthermore, in considerations of structure strength and insulating property, sufficient spacing is required between conductive holes and between conductive holes and copper wires. As a result, the overall area of the circuit board would need to increase, and thus the cost would be undesirably raised and the compactness of the product would be adversely affected.